EP-A-168892 describes an endothermic steam reforming reaction, which is carried out in a fixed bed situated in at least one pipe in which a temperature of between 800 and 950° C. is maintained by routing at least part of the hot product gas from a partial oxidation reaction along the pipe(s). According to this publication the combined partial oxidation and endothermic production of synthesis gas result in a better yield of synthesis gas, an increased H2/CO ratio, a lower usage of oxygen per m3 of synthesis gas product obtained and a lower capital cost of the plant for the production of CO and H2-containing gas mixtures (as compared to partial oxidation).
A reactor and process for performing a steam reforming reaction is described in DE-A-3345088. This publication describes a reactor vessel for performing a steam reforming reaction starting from a natural gas feedstock. The vessel consisted of a tube sheet from which a plurality of tubes filled with a suitable catalyst extended into the vessel. The required heat of reaction is provided by passing the hot effluent of a partial oxidation reaction of natural gas at the exterior of the reactor tubes in the vessel. Such steam reformer reactors are also referred to as so-called convective steam reformer reactors.
A disadvantage of the known reactor vessel design is that fouling may occur at the exterior surface of the reactor tubes. This fouling will result in a less favorable heat exchange between hot gas and the catalyst bed and in time result in a less efficient operation. Short run times will result due to frequent shutdowns in order to remove the deposits. Fouling is especially a problem when the hot effluent of a partial oxidation reaction is used. This effluent is especially suited for providing the required heat due to its high temperatures. However, the soot present in this effluent will cause the above fouling problems.
EP-A-983964 describes a convective steam reforming reactor vessel, wherein the vessel is provided with a plurality of reactor tubes containing a catalyst bed. Around the reactor tubes an annular sleeve is provided to transport a hot effluent of an auto thermal reformer (ATR). By indirect heat exchange between this hot effluent and the reactants passing through the catalyst bed the steam reforming reaction can take place.
WO-A-0137982 discloses a reformer tube of the so-called double-tube configuration of a steam reformer reactor. The double-tube configuration consists of a reactor tube provided with a catalyst bed in which bed an inner return tube is provided for passage of the reactants being discharged from said catalyst bed. The double-tube configuration is described in more detail in U.S. Pat. No. 4,690,690 according to WO-A-0137982. The inner tube as disclosed in WO-A-0137982 has a non-circular cross-section.
WO-A-8801983 discloses a convective steam reforming reactor vessel wherein the hot gas, which is used to heat the reactor tubes, is obtained by burning heating gas in a lower part of the vessel.
It would be useful to provide a reactor design for a convective steam reformer, which is suitable to make use of a hot gaseous medium, which may cause fouling, as for example the effluent of a partial oxidation, as the heating medium. It would also be useful to provide a process for the preparation of a mixture of hydrogen and carbon monoxide by a process involving the combination of partial oxidation and steam reforming wherein the problems regarding fouling as described above are minimized.